Abstract : Mass spectra of ammonia (NH3) were obtained after charge exchange with slow positive ions. The intensities of the peaks were plotted as a function of the energy absorbed by the molecule during the charge exchange. Two regions with very low probability for ionization exist between 12.5 and 15 eV and around 21 eV. Integration of the breakdown graph gives a mass spectrum in agreement with that obtained in electron impact. The ion-molecule reactions were studied using the pressure dependence of the ion-currents in the pressure range mm Hg. The order and mechanisms of the formation of the product ions are determined at different energies absorbed by the molecule during the charge exchange. The structure and dissociation of ammonia are discussed. (Author)